In fiber data distributed interface (FDDI) systems, great care is needed in order to reduce baseline wander or DC bias over time, The FDDI system is a serial communication system wherein serial bits are serially transmitted. In FDDI twisted pair systems that conform with the ANSI standard, this serial transmission over a period of time may contain an imbalance among the transmitted voltage levels resulting in a DC offset or base line wander over that period of time.
In order to reduce emissions in twisted pair FDDI the data frame is first scrambled by a psuedorandom number generator and then an MLT3 encoding scheme is used. The scrambled MLT3 encoded bit stream transmits positive and negative values in a toggled manner with no regard for transmission history. Study showed that the use of 4B5B (a technique for converting 4-bit data elements to 5-bit data elements with error tolerance and framing character requirements taken into account), scrambling, and MLT3 together had the potential to create baseline wander or DC bias. In fact, all known FDDI systems failed when certain valid sequences where applied to them using the 4B5B, scrambling, and MLT3 scheme because baseline wander was too large over time to allow error-free data recovery.
Therefore, a better encoding scheme is needed for twisted pair FDDI systems.